We have examined the ability of Helianthus hypocotyl segments as well as segments from a variety of other species to elongate in response to HI and to secrete HI in response to auxin and fusicoccin. In all cases a positive response was obtained when the cuticular barrier was abraded with carborundum. Removal of the cuticular barrier by peeling prevented detection of both auxin-induced elongation and HI secretion. Fusicoccin-induced growth and acid secretion are not prevented by peeling. These results suggest considerable tissue selectivity with respect to auxin action but considerably less specificity with respect to fusicoccin. It seems likely that in many dicots auxin-enhanced proton secretion and elongation are controlled by the epidermis and/or closely associated cell layers. The data presented in this paper provide further support for the acid growth theory of auxin action.